There is a widely known method in which a single layer made of a polyvinyl alcohol type resin (hereinafter referred to as “PVA type resin”) formed in a film shape is subjected to dyeing and stretching to produce a polarizing film comprising a PVA type resin layer, wherein molecules of the PVA type resin are oriented in the direction of the stretching, and a dichroic material is absorbed (impregnated) in the PVA type resin in a molecularly oriented state. The thickness of a polarizing film to be obtained by the above conventional method using a PVA type resin single-layer film is in the range of about 15 to 35 μm. The conventional method makes it possible to obtain a polarizing film having the following optical characteristics of a single layer transmittance of 42% or more, and a polarization rate of 99.95% or more. Currently, polarizing films produced by the conventional method are used in optical display devices for televisions, mobile phones, personal digital assistants, and other appliances.
However, since the PVA type resins are hydrophilic and highly hygroscopic, a polarizing film produced using a PVA type resin is sensitive to changes in temperature and humidity, and more likely to expand and contract due to changes in surrounding environments, which is liable to cause the occurrence of crack. For this reason, a conventional typical polarizing film has been used as an optical film laminate prepared by laminating a triacetylcellulose (TAC) film having a thickness of 40 to 80 μm and serving as a protection film, on each of opposite surfaces thereof.
Another problem when using a conventional polarizing film consisting of a PVA type resin layer is that expansion and contraction caused by environmental changes during use will produce stress in an adjacent member to which the polarizer film is joined, and thereby cause deformation, such as warp, in the adjacent member.
However, even in the optical film laminate where a triacetylcellulose (TAC) film serving as a protection film is laminated on each of opposite surfaces of a polarizing film, in cases where a single-layer polarizing film is used therein, there is a limit to thinning of the polarizing film. Thus, expansion and contraction forces of the polarizing film become unignorable, and it is difficult to completely suppress the influence of expansion and contraction of the polarizing film, so that a certain level of expansion and contraction will inevitably occur in the optical film laminate including the polarizing film. If expansion or contraction occurs in such an optical film laminate including a polarizing film, stress arising from the expansion or contraction will cause deformation, such as warp, in an adjacent member. This deformation, even if it is small, leads to the occurrence of non-uniformity of display in a liquid-crystal display device. To suppress the occurrence of such non-uniformity of display, design considerations should be made to carefully select the material for each member to be used in the optical film laminate including the polarizing film. Further, the contraction stress of the polarizing film will cause peeling or the like of the optical film laminate from a liquid-crustal panel. Thus, a high-adhesion adhesive is required to join the optical film laminate to the liquid-crystal display panel. However, the use of such a high-adhesion adhesive gives rise to a problem of difficulty in re-working which is an operation of, when the presence of an optical defect is found in a polarizing film of an optical film laminate laminated to a liquid-crystal display panel through a subsequent inspection, peeling the optical film laminate from the liquid-crystal display panel and laminating another optical film laminate to the liquid-crystal display panel. This is one technical problem in a polarizing film to be obtained by the conventional method using a PVA type resin single-layer formed in a film shape.
The problem causes a growing demand for a new polarizing film production method, as an alternative to the conventional polarizing film production method using a PVA type resin single-layer, and being incapable of achieving a sufficient level of thinning of a polarizing film due to the above problem. Specifically, it is virtually impossible to produce a polarizing film having a thickness of 10 μm or less by the conventional method using a PVA type resin single-layer formed in a film shape. This is because, in producing a polarizing film using a film-shaped PVA type resin single-layer, if the thickness of the PVA type resin single-layer is excessively reduced, dissolution and/or breaking is likely to occur in a PVA type resin layer in a dyeing step and/or a stretching step, which makes it impossible to form a polarizing film having a uniform thickness.
To address the problem, there has been proposed a method designed such that a PVA type resin layer is applied and formed on a thermoplastic resin substrate, and the PVA type resin layer formed on the resin substrate is stretched together with the resin substrate, and subjected to dyeing, so as to produce a polarizing film significantly thinner than the polarizing film obtained by the conventional method. This polarizing film production method using a thermoplastic resin substrate is noteworthy in that it provides a possibility of producing a polarizing film more uniformly than the polarizing film production method using a PVA type resin single-layer.
For example, Japanese Patent JP 4279944B (Patent Document 1) discloses a polarizing plate production method which comprises steps of forming a polyvinyl alcohol resin layer having a thickness of 6 μm to 30 μm, on one of opposite surfaces of a thermoplastic resin film by a coating process, stretching the polyvinyl alcohol resin layer at a stretching ratio of 2.0 to 5.0 in such a manner that the polyvinyl alcohol resin layer is formed as a transparent coating element layer to thereby form a composite film consisting of two layers including the thermoplastic resin film and the transparent coating element layer, laminating an optical transparent resin film layer on the side of the transparent coating element layer of the composite film consisting of the two layers, through a bonding agent, peeling and removing the thermoplastic resin film, and dyeing and fixing the transparent coating element layer in such a manner that the transparent coating element layer is formed as a polarizing element layer. A polarizing plate to be obtained by this method has a two-layer structure consisting of the optical transparent resin film layer and the polarizing element layer. According to the description of the Patent Document 1, the polarizing element has a thickness of 2 to 4 μm.
The method disclosed in the Patent Document 1 is designed to perform a stretching under an elevated temperature by a uniaxial stretching process, wherein the stretching ratio is restricted to the range of 2.0 to 5.0, as mentioned above. As for the reason why the stretching ratio is restricted to 5.0 or less, the Patent Document 1 explains that a stretching at a high stretching ratio of greater than 5.0 makes it extremely difficult to maintain stable production. Specifically, there is described that the ambient temperature during a stretching is set to 55° C. in cases where ethylene-vinyl acetate copolymer is used as the thermoplastic resin film, to 60° C. in cases where non-stretched polypropylene is used as the thermoplastic resin film, or to 70° C. in cases where non-stretched nylon is used as the thermoplastic resin film. The method disclosed in the Patent Document 1 employs a uniaxial stretching process in air under elevated temperature. Further, as described in the Patent Document 1, the stretching ratio is restricted to 5.0 or less. Thus, a polarizing film having an extremely small thickness of 2 to 4 μm, to be obtained by this method, is not enough to satisfy optical characteristics desired for a polarizing film to be used, for example, in an optical display device such as a liquid-crystal television, or an optical display device using an organic EL display element.
The method of forming a polarizing film with steps of forming a PVA type resin layer on a thermoplastic resin substrate by a coating process, and stretching the PVA type resin layer together with the substrate is also disclosed in Japanese Patent Laid-Open Publication JP 2001-343521A (Patent Document 2) and Japanese Patent Laid-Open Publication JP 2003-043257A (Patent Document 3). The methods disclosed in the Patent Documents 2 and 3 are designed such that a laminate consisting of a thermoplastic resin substrate and a PVA type resin layer applied on the substrate is subjected to a uniaxial stretching at a temperature of 70° C. to 120° C., in cases where the substrate is made of a non-crystallizable polyester resin. Then, the PVA type resin layer molecularly oriented by the stretching is subjected to dyeing to allow a dichroic material to be impregnated therein. In the Patent Document 2, there is described that the uniaxial stretching may be a longitudinal uniaxial stretching or may be a transverse uniaxial stretching. Differently, in the Patent Document 3, a method is described in which the transverse uniaxial stretching is performed, and, during or after the transverse uniaxial stretching, contracting the length in the direction perpendicular to the direction of the stretching by a specific amount. In both of the Patent Documents 2 and 3, there is described that the stretching ratio is typically set to about 4.0 to 8.0. Further, there is described that the thickness of a polarizing film to be obtained is in the range of 1 to 1.6 μm.
In the Patent Documents 2 and 3, although there is described that the stretching ratio is typically set to 4.0 to 8.0, since the Patent Documents 2 and 3 adopt an elevated temperature in-air stretching process, it is considered that the stretching ratio is limited to 5 as described, for example, in the Patent Document 1. Neither of these describes a specific technique for achieving a stretching ratio of greater than 5.0 by the elevated temperature in-air stretching process. In fact, in Examples described in the Patent Documents 2 and 3, only a stretching ratio of 5.0 and a stretching ratio of 4.5 are described, respectively, in the Patent Document 2 and the Patent Document 3. Through additional tests on the methods disclosed in the Patent Documents 2 and 3, the inventors of the present invention have ascertained that it is impossible to adequately perform a stretching at a stretching ratio of greater than 5.0 by the methods disclosed therein. Therefore, it should be understood that only a stretching ratio of 5.0 or less is substantially disclosed in the Patent Documents 2 and 3. As with the Patent Document 1, the polarizing film to be obtained by the method disclosed in each of the Patent Documents 2 and 3 is not enough to satisfy optical characteristics desired for a polarizing film to be used, for example, in an optical display device such as a liquid-crystal television.
U.S. Pat. No. 4,659,523 (Patent Document 4) discloses a polarizing film production method which comprises subjecting a PVA type resin layer coated on a polyester film to a uniaxial stretching together with the polyester film. This method 4 is intended to form the polyester film serving as a substrate of the PVA type resin layer in such a manner as to have optical characteristics allowing the polyester film to be used together with a polarizing film, but it is not intended to produce a polarizing film comprising a PVA type resin layer and having a small thickness and excellent optical characteristic. Specifically, the method disclosed in the Patent Document 4 is no more than a technique of improving optical characteristics of a polyester resin film to be stretched together with a PVA type resin layer to be formed as a polarizing film. A polarizer material production method having the same object is also disclosed in Japanese Patent Publication JP 08-012296B (Patent Document 5).
Generally, the aforementioned optical film laminate having a TAC film laminated on each of opposite surfaces of a polarizing film is used in such a manner that it is laminated to an optical display panel, such as a liquid-crystal display panel. There has already been proposed a continuous lamination apparatus designed such that a carrier film-attached optical film laminate prepared by attaching a carrier film to the optical film laminate through an adhesive layer is cut into a plurality of laminate sheets each having a length conforming to a dimension of each optical display panel, while being continuously fed in a lengthwise direction thereof, and the laminate sheets are sequentially laminated to respective ones of the optical display panels, as disclosed, for example, in JP 4361103B (Patent Document 6), JP 4377961B (Patent Document 7), JP 4377964B (Patent Document 8), JP 4503689B (Patent Document 9), JP 4503690B (Patent Document 10) and JP 4503691B (Patent Document 11).
An optical film laminate continuous lamination apparatus disclosed in the above Patent Documents comprises a slit forming mechanism for forming a plurality of slits in a carrier film-attached optical film laminate being continuously fed, at lengthwise intervals corresponding to one of long and short sides of an optical display panel to which the optical film laminate is to be laminated, to extend in a direction perpendicular to the lengthwise direction. The slit forming mechanism is adapted to form each of the slits to extend from a surface of the carrier film-attached optical film laminate on a side opposite to the carrier film, in a width direction of the laminate, up to a depth reaching an interface between the carrier film and the adhesive layer. Such a slit forming technique is called “half-cutting”. According to the half-cutting, an optical film laminate sheet having a length corresponding to a dimension of one of the long and short sides of the optical display panel is formed between two of the slits located adjacent to each other in the lengthwise direction of the carrier film-attached optical film laminate. In this case, a width of the optical film laminate is set to a value corresponding to a dimension of a remaining one of the long and short sides of the optical display panel.
The optical film laminate continuous lamination apparatus further comprises a panel feeding mechanism for sequentially feeding optical display panels to a lamination position. The optical film laminate sheets are fed toward the lamination position in synchronization with respective ones of the optical display panels being sequentially fed to the lamination position. A carrier-film peeling mechanism is provided just before the lamination position. The carrier-film peeling mechanism is operable to peel each of the optical film laminate sheets, while allowing the adhesive layer to be left on the side of the optical film laminate sheet. Then, the optical film laminate sheet peeled from the carrier film is fed to be superimposed on the optical display panel fed to the lamination position. A laminating mechanism, such as a pair of laminating rollers, is provided at the lamination position. The laminating mechanism is operable to laminate the optical film laminate sheet to the optical display panel fed to the lamination position, through the adhesive layer.
The carrier-film peeling mechanism comprises a peeling plate having an edge portion formed in a shape for causing the carrier film peeled from the optical film laminate sheets to be folded back at an acute angle. The optical film laminate sheet is released from the carrier film and fed to the lamination position, without changing a moving direction thereof.